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Buffered-cluster method for hybridization of density-functional theory and classical molecular dynamics: Application to stress-dependent reaction of H2O on nanostructured Si
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A hybrid density-functional-theory and molecular-dynamics simulation scheme was proposed [Ogata, Comput. Phys. Commun. 149, 30 (2002)] in which a total atomistic system is partitioned, in real space, into the quantum (QM) region whose electronic structure is calculated with the density-functional theory and the classical (CL) region treated with the interatomic potential for the molecular dynamics. In the scheme, the link-atom method that uses hydrogen atoms for termination of the QM atoms is adopted to couple the QM and CL regions mechanically. A proper choice of the QM region that retains the original atomic configuration is limited in the link-atom method. In this paper we propose a coupling method, called the buffered-cluster method, with the introduction of buffer atoms to minimize possible effects arising from the finiteness of the size of the QM region. The buffered-cluster method is applicable to any reasonable choice of the QM region in a wide range of ceramics and semiconductor materials. The accuracy of the buffered-cluster method is analyzed by applying it to crystalline Si and alumina systems, to find little differences around the QM-CL boundaries in both relaxed configuration of the atoms and recoil forces on them due to their trial displacements. The insensitivity of the atomic forces to the choice of the QM region in the buffered-cluster method makes it possible to rechoose the QM region adaptively during the hybrid simulation run for fast computation. The hybrid simulation scheme with the buffered-cluster method is applied to analyze adsorption and dissociation processes of an H2O molecule on a notched Si-slab system with or without strains, in which the H2O interacts with the notch-bottom facet of Si(100)-(2×1) dimer structure. The QM region is chosen in the system to cover the reaction region. Energy variations along the reaction paths show that the adsorption energy and the dissociation barrier of the H2O molecule on the Si(100) facet in the system are sensitive to the stain. The adsorption energy decreases substantially, while the dissociation barrier with H transferred to a nearby dimer increases, when the system is stretched. We perform hybrid simulation runs with the buffered-cluster method to study the adsorption and dissociation dynamics of the H2O molecule with the facet of Si(100)-(2×1) in the nanostructured Si system at both stretched and unstrained conditions, in which the QM region is rechosen dynamically to trace the reaction atoms. The probability of the H2O dissociation and the following H-transfer path depend significantly on the strain applied to the system and on the initial conditions of the molecule.
Buffered-cluster method for hybridization of density-functional theory and classical molecular dynamics: Application to stress-dependent reaction of H2O on nanostructured Si
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